aaaaa,
It all comes down to how many times you install and uninstall programs or how often you download large files from the Internet and then delete them. (If you do not do so often, you can go months without Defraging.)
Now the time it takes to Defrag depends on the speed of your comp, the size of the Hard Drive, and the amount of Data on it.
Personally I do not Defrag, unless I am about to benchmark my Drives. WinXP does a really good job of Disk Management and unless you see an issue where each time you run a program, the Hard Drive runs a long time, I would not worry about it.
Now if you wanted to be really disciplined, then once a week or once a month would more than adequate.
If you Defrag regularly it will take less time per Defrag then if you go for long periods without Defragging. You could always leave the Drive Defragging over night and by morning you would be ready to go when you get up.
Take Care,
Richard